Semiconductor devices can be made in a layer or film of silicon on a quartz or glass substrate, for example. This technology is in use in the manufacture of image sensors and active-matrix liquid-crystal display (AMLCD) devices. In the latter, in a regular array of thin-film transistors (TFT) on an appropriate transparent substrate, each transistor serves as a pixel controller. In commercially available AMLCD devices, the thin-film transistors are formed in hydrogenated amorphous silicon films (a-Si:H TFTs).
In the interest of enhanced switching characteristics of TFTs, polycrystalline silicon has been used instead of amorphous silicon. A polycrystalline structure can be obtained by excimer-laser crystallization (ELC) of a deposited amorphous or microcrystalline silicon film, for example.
However, with randomly crystallized poly-silicon, the results remain unsatisfactory. For small-grained poly-silicon, device performance is hampered by the large number of high-angle grain boundaries, e.g., in the active-channel region of a TFT. Large-grained poly-silicon is superior in this respect, but significant grain-structure irregularities in one TFT as compared with another then result in non-uniformity of device characteristics in a TFT array.